Exploration of genomic DNA has long been desired by the scientific, in particular medical, community. Genomic DNA holds the key to identification, diagnosis and treatment of diseases such as cancer and Alzheimer's disease. In addition to disease identification and treatment, exploration of genomic DNA may provide significant advantages in plant and animal breeding efforts, which may provide answers to food and nutrition problems in the world.
Many diseases are known to be associated with specific genetic components, in particular with polymorphisms in specific genes. The identification of polymorphisms in large samples such as genomes is at present a laborious and time-consuming task. However, such identification is of great value to areas such as biomedical research, developing pharmacy products, tissue typing, genotyping and population studies.
Markers, i.e. genetic markers, have been used for a very long time as a genetic typing method, i.e. to connect a phenotypic trait to the presence, absence or amount of a particular part of DNA (gene). One of the most versatile genetic typing technologies is AFLP, already around for many years and widely applicable to any organism (for reviews see Savelkoul et al. J. Clin. Microbiol, 1999, 37(10), 3083-3091; Bensch et al. Molecular Ecology, 2005, 14, 2899-2914)
The AFLP technology (Zabeau & Vos, 1993; Vos et al., 1995) has found widespread use in plant breeding and other field since its invention in the early nineties. This is due to several characteristics of AFLP, of which the most important is that no prior sequence information is needed to generate large numbers of genetic markers in a reproducible fashion. In addition, the principle of selective amplification, a cornerstone of AFLP, ensures that the number of amplified fragments can be brought in line with the resolution of the detection system, irrespective of genome size or origin.
Detection of AFLP fragments is commonly carried out by electrophoresis on slab-gels (Vos et al., 1995) or capillary electrophoresis (van der Meulen et al., 2002). The majority of AFLP markers scored in this way represent (single nucleotide) polymorphisms occurring either in the restriction enzyme recognition sites used for AFLP template preparation or their flanking nucleotides covered by selective AFLP primers. The remainder of the AFLP markers are insertion/deletion polymorphisms occurring in the internal sequences of the restriction fragments and a very small fraction on single nucleotide substitutions occurring in small restriction fragments (<approximately 100 bp), which for these fragments cause reproducible mobility variations between both alleles; these AFLP markers can be scored co-dominantly without having to rely on band intensities.
In a typical AFLP fingerprint, the AFLP markers therefore constitute the minority of amplified fragments (less than 50 percent but often less than 20 percent), while the remainder are commonly referred to as constant AFLP fragments. The latter are nevertheless useful in the gel scoring procedure as they serve as anchor points to calculate fragments mobilities of AFLP markers and aid in quantifying the markers for co-dominant scoring. Co-dominant scoring (scoring for homo- or heterozygosity) of AFLP markers currently is restricted to the context of fingerprinting a segregating population. In a panel of unrelated lines, only dominant scoring is possible.
Although the throughput of AFLP is very high due to high multiplexing levels in the amplification and detection steps, the rate limiting step is the resolving power of electrophoresis. Electrophoresis allows unique identification of the majority of amplified fragments based on the combination of restriction enzyme combinations (EC), primer combinations (PC) and mobility, but ideally, the detection system should be capable of determining the entire sequence of the amplified fragments to capture all polymorphisms.
Detection by sequencing instead of mobility determination will increase throughput because:
1) polymorphisms located in the internal sequences will be detected in most (or all) amplified fragments; this will increase the number of markers per PC considerably.
2) no loss of AFLP markers due to co-migration of AFLP markers and constant bands.
3) co-dominant scoring does not rely on quantification of band intensities and is independent of the relatedness of the individuals fingerprinted.
So far, detection of AFLP markers/sequences by sequencing has not been economically feasible due to, among other limitations, cost limitations of Sanger dideoxy sequencing technology and other conventional sequencing technologies.
Accordingly, it is one of the goals of the present invention to provide for economically feasible methods for the detection of AFLP markers or other genetic markers such as SNP markers based on sequencing.
An important problem further associated with detection of a collection of AFLP or SNP containing fragments via sequencing for genotyping (i.e. diagnostic) purposes is that of sampling variation. Specifically, this means that when a collection of fragments is analyzed and particular fragments are not observed, one has to make sure that this is not due to the fact that the fragments involved were not sampled at the detection step, although they are present in the fragment mixture, because this would lead to false-negative scoring of the marker. This limitation does not apply to detection by electrophoresis because position information on the gel is available.
Accordingly, it is one of the further goals of the present invention provide a method that solves the problem of sample variation or at least reduces the errors caused by sample variation to an acceptable minimum.